Austenitic Stainless Steel Material

ABSTRACT

There is provided an austenitic stainless steel material having a consistent high-strength across the overall length of the steel material, which has a chemical composition consisting of, in mass percent: C: 0.10% or less, Si: 1.0% or less, Mn: 3 to 8%, P: 0.05% or less, S: 0.03% or less, Ni: 10 to 20%, Cr: 15 to 30%, N: 0.20 to 0.70%, with the balance being Fe and impurities, the austenitic stainless steel material having a grain size number of 6.0 or greater, the grain size number conforming to ASTM E 112 tensile strength of the austenitic stainless steel material is 800 MPa or more, and the difference between the maximum value and the minimum value of the tensile strength is 50 MPa or smaller. The number of alloy carbo-nitrides having a circle equivalent diameter of larger than 1000 nm in the steel is 10/mm2 or more.

TECHNICAL FIELD

The present invention relates to a stainless steel material more specifically to an austenitic stainless steel material.

BACKGROUND ART

Recent years have seen the progress of lively studies for the practical application of transportation equipment that utilizes hydrogen in place of fossil fuels as energy. For example, fuel cell powered vehicles that run on hydrogen as fuel, and hydrogen stations where fuel cell powered vehicles are supplied with hydrogen, have been under development.

When stainless steel is used for hydrogen stations, the stainless steel is placed in a high-pressure hydrogen gas environment. For this reason, an excellent strength is demanded of a stainless steel used for hydrogen stations.

International Application Publication No. WO 2012/132992 (Patent Literature 1), International Application Publication No. WO 2004/083476 (Patent Literature 2), International Application Publication No. WO 2004/083477 (Patent Literature 3), and International Application Publication No. WO 2004/111285 (Patent Literature 4) propose stainless steels that are used in high-pressure hydrogen environments and have high strengths.

The austenitic stainless steel for high-pressure hydrogen gas disclosed in Patent Literature 1 contains, in mass percent. C: 0.10% or less, Si: 1.0% or less, Mn: 3% or more and less than 7%, Cr: 15 to 30%, Ni: 10% or more and less than 17%, Al: 0.10% or less, N: 0.10 to 0.50%, and at least one of V: 0.01 to 1.0% and Nb: 0.01 to 0.50%, with the balance being Fe and impurities, of which impurities P accounts for 0.0050% or less, and S accounts for 0.050% or less, wherein the austenitic stainless steel has a tensile strength of 800 MPa or more, a grain size number (ASTM E 112) is 8 or greater, and the austenitic stainless steel contains an alloy carbo-nitride with a maximum diameter of 50 to 1000 nm at 0.4/μm² or more in cross section observation.

The stainless steel for hydrogen gas disclosed in Patent Literature 2 contains, in mass percent, C: 0.02% or less, Si: 1.0% or less, Mn: 3 to 30° %, Cr: larger than 22% to 30%, Ni: 17 to 30%, V: 0.001 to 1.0%, N: 0.10 to 0.50%/6, and Al: 0.10% or less, with the balance being Fe and impurities, of which impurities P accounts for 0.030% or less, S accounts for 0.005% or less, and Ti, Zr, and Hf each account for 0.01% or less, wherein the contents of Cr, Mn, and N satisfies 5Cr+3.4Mn≤500N.

The stainless steel for high-pressure hydrogen gas disclosed in Patent Literature 3 contains, in mass percent, C: 0.04% or less. Si: 1.0% or less, Mn: 7 to 30%, Cr: 15 to 22%, Ni: 5 to 20%, V: 0.001 to 1.0%, N: 0.20 to 0.50%, and Al: 0.10% or less, with the balance being Fe and impurities, of which impurities P accounts for 0.030% or less, S accounts for 0.005% or less, and Ti, Zr, and Hf each account for 0.01% or less, wherein the contents of Cr, Mn, and N satisfies 2.5Cr+3.4Mn≤300N.

The austenitic stainless steel for hydrogen gas disclosed in Patent Literature 4 has a chemical composition containing, in mass percent, C: 0.10% or less, Si: 1.0% or less, Mn: 0.01 to 30%, P: 0.040% or less, S: 0.01% or less, Cr: 15 to 30%, Ni: 5.0 to 30%, sol. Al: 0.10% or less, and N: 0.001 to 0.30%, with the balance being Fe and impurities, wherein the austenitic stainless steel includes a micro-structure in which an X-ray integrated intensity I (111) on a cross section along a direction perpendicular to a processing direction is five times or less that in a random orientation, and an X-ray integrated intensity I (220) on a cross section along the processing direction satisfies I(220)/I(111)≤10.

CITATION LIST Patent Literature

-   Patent Literature 1: International Application Publication No. WO     2012/132992 -   Patent Literature 2: International Application Publication No. WO     2004/083476 -   Patent Literature 3: International Application Publication No. WO     2004/083477 -   Patent Literature 4: International Application Publication No. WO     2004/111285

SUMMARY OF INVENTION Technical Problem

Now, a stainless steel to be used for hydrogen stations is required to have not only an excellent strength but also suppressed variation in strength. Stainless steels disclosed in Patent Literature 1 to Patent Literature 4 mentioned above have strengths of 700 MPa or more even after solution treatment is performed, and the stainless steel of Patent Literature 4 has a high strength by being subjected to solution treatment and cold working. However, these Patent Literatures give no consideration to variation in strength. Even the stainless steels described in Patent Literature 1 to Patent Literature 4 mentioned above may show large variations in strength, failing to provide consistent high-strengths.

An objective of the present invention is to provide an austenitic stainless steel material having a consistent high-strength across the overall length of the steel material.

Solution to Problem

An austenitic stainless steel material according to the present embodiment has a chemical composition consisting of, in mass percent, C: 0.10% or less, Si: 1.0% or less, Mn: 3 to 8%, P: 0.05% or less, S: 0.03% or less, Ni: 10 to 20%, Cr: 15 to 30%, N: 0.20 to 0.70%, Mo: 0 to 5.0%, V: 0 to 0.5%, and Nb: 0 to 0.5%, with the balance being Fe and impurities, the austenitic stainless steel material having a grain size number of 6.0 or greater, the grain size number conforming to ASTM E 112. The tensile strength of the austenitic stainless steel material is 800 MPa or more, and the difference between the maximum value and the minimum value of the tensile strength is 50 MPa or less. The number of alloy carbo-nitrides having a circle equivalent diameter of larger than 1000 nm in the steel is 10/mm² or more.

Advantageous Effects of Invention

The austenitic stainless steel material according to the present embodiment has a consistent high-strength across the overall length of the steel material.

DESCRIPTION OF EMBODIMENTS

The present inventors conducted investigations and studies on the high strengthening of an austenitic stainless steel material and variation in strength across the overall length of the steel material, and obtained the following findings.

(A) Methods for increasing the strength include solid-solution strengthening using N and grain refinement. The austenitic stainless steel of the present embodiment contains 0.20 to 0.70% of N, and the strength thereof is increased through the solid-solution strengthening. The strength is further increased by grain refining on grains.

(B) The variation in strength across the overall length of the steel material is attributable to the grain size of the steel material. With a smaller variation in grain size in the steel material, the variation in strength can be reduced. Specifically, when the grain size number based on the ASTM E 112 is 6.0 or greater, and the difference between the maximum value and the minimum value of the grain size number (hereafter, referred to as a grain size difference ΔGS) across the overall length of the steel material is 1.5 or smaller, the difference between the maximum value and the minimum value of the tensile strength (hereafter, referred to as a strength difference ΔTS) across the overall length of the steel material becomes 50 MPa or less, which enables the variation in strength to be suppressed sufficiently.

(C) To suppress the variation in strength, controlling temperature variation in a starting material in the hot working is effective. Variation in grain size is brought most remarkably in the hot working. In the starting material, the introduction amount of strain differs between a portion at a lower temperature and a portion at a higher temperature. The difference in the introduction amount of strain causes the difference in how grains are refined in recrystallization. As a result, the variation in grain size is increased. Therefore, it is preferable that the temperature variation in a starting material in hot working is small.

Specifically, in the starting material, if the difference between the temperature of a portion for which the hot working is first completed, at the completion of the working (hereafter, referred to as an initial temperature) and the temperature of a portion for which the hot working is last completed, at the completion of the working (hereafter, referred to as an end temperature) (temperature difference ΔT) is 100° C. or less, the grain size difference ΔGS can be controlled to 1.5 or smaller. Consequently, it is possible to control a strength difference ΔTS to 50 MPa or less.

(D) By performing heat treatment on a steel material to cause coarse alloy carbo-nitrides to precipitate, precipitation strengthening occurs, further increasing the strength of the steel material. When the grain size number of a steel material is 6.0 or greater, and the number of alloy carbo-nitrides having circle equivalent diameters of larger than 1000 nm (hereafter, referred to as a coarse alloy carbo-nitrides) is 10/mm² or more in the steel, a tensile strength of 800 MPa or more is obtained. By performing the heat treatment at a heat treatment temperature set at 930° C. to less than 1000° C., it is possible to obtain 10/mm² or more of the coarse alloy carbo-nitrides.

Here, the alloy carbo-nitrides contain Cr, V, Nb, Mo, W, Ta, or the like as a main component, and mean Cr₂N, Z phase, namely Cr(Nb, V)(C, N) and MX (M: Cr, V, Nb, Mo, W, Ta, etc., X: C, N). The “main component” means that the component accounts for 40% or more in mass percent. In addition, the alloy carbo-nitrides in the present invention include those of which contents of C (carbon) are extremely low, namely nitrides. The alloy carbo-nitrides according to the present invention include carbides.

An austenitic stainless steel material according to the present embodiment that is completed based on the above findings has a chemical composition consisting of, in mass percent, C: 0.10% or less, Si: 1.0% or less, Mn: 3 to 8%, P: 0.05% or less, S: 0.03% or less, Ni: 10 to 20%, Cr: 15 to 30%, N: 0.20 to 0.70%, Mo: 0 to 5.0%, V: 0 to 0.5%, and Nb: 0 to 0.5%, with the balance being Fe and impurities, the austenitic stainless steel material having a grain size number of 6.0 or greater, the grain size number conforming to ASTM E 112. The tensile strength of the austenitic stainless steel material is 800 MPa or more, and the difference between the maximum value and the minimum value of the tensile strength is 50 MPa or less. The number of alloy carbo-nitrides having a circle equivalent diameter of larger than 1000 nm in the steel is 10/mm² or more.

The above chemical composition may contain one, or two or more elements selected from the group consisting of, in mass percent, Mo: 1.5 to 5.0%, V: 0.1 to 0.5%, and Nb 0.1 to 0.5%.

In the above austenitic stainless steel material, the difference between the maximum value and the minimum value of the above grain size number is 1.5 or smaller.

The above austenitic stainless steel material is, for example, a steel pipe, a steel bar, or a wire rod.

Hereafter, the austenitic stainless steel material of the present embodiment will be described in detail. The sign “%” following each element means mass percent unless otherwise noted.

[Chemical Composition]

The austenitic stainless steel material of the present embodiment has a chemical composition that consists of the following elements.

C: 0.10% or Less

Carbon (C) is unavoidably contained. C stabilizes austenite in an fcc structure, which hardly causes hydrogen brittleness. In addition, C combines with Cr or the like, causing precipitation strengthening to increase the strength of steel. However, an excessively high content of C results in the precipitation of carbides in grain boundaries, decreasing the toughness of steel. Consequently, the content of C is 0.10% or less. An upper limit of the content of C is preferably 0.08%, more preferably 0.06%. In addition, a preferable lower limit of the content of C to stabilize austenite is 0.005%.

Si: 1.0% or Less

Silicon (Si) combines with Ni and Cr to form intermetallic compounds. In addition, Si contributes to the growth of intermetallic compounds such as a sigma phase (σ phase). These intermetallic compounds decrease the hot workability of steel. Therefore, the content of Si is 1.0% or less. An upper limit of the content of Si is preferably 0.8%. From the viewpoint of deoxidizing steel, a preferable lower limit of the content of Si is 0.2%.

Mn: 3 to 8%

Manganese (Mn) stabilizes austenite and suppresses the generation of martensite, which has a high susceptibility to hydrogen brittleness. In addition, Mn combines with S to form MnS, increasing the machinability of steel. An excessively low content of Mn results in failure to provide the effects described above. In contrast, an excessively high content of Mn results in a decrease in the ductility and the hot workability of steel. Consequently, the content of Mn is 3 to 8%. A lower limit of the content of Mn is preferably 4.0%, more preferably 5.0%. An upper limit of the content of Mn is preferably 6.0%, more preferably 5.9%.

P: 0.05% or Less,

Phosphorus (P) is an impurity. P decreases the hot workability and the toughness of steel. Therefore, the content of P is 0.05% or less. An upper limit of the content of P is preferably 0.045%, more preferably 0.035%, still more preferably 0.020%. The content of P is preferably as low as possible.

S: 0.03% or Less

Sulfur (S) combines with Mn to form MnS, increasing the machinability of steel. However, an excessively high content of S results in a decrease in the toughness of steel. Therefore, the content of S is 0.03% or less. An upper limit of the content of S is preferably 0.02%, more preferably 0.01%. The content of S is preferably as low as possible.

Ni: 10 to 20%

Nickel (Ni) stabilizes austenite. In addition, Ni increases the ductility and the toughness of steel. An excessively low content of Ni results in failure to provide the effects described above. In contrast, an excessively high content of Ni results in saturation of the effects described above, increasing production costs. Consequently, the content of Ni is 10 to 20%. A lower limit of the content of Ni is preferably 11.5%, more preferably 12.0%. An upper limit of the content of Ni is preferably 13.5%, more preferably 13.4%.

Cr: 15 to 30%

Chromium (Cr) increases the corrosion resistance of steel. In addition, Cr combines with N through heat treatment to form alloy carbo-nitrides such as Cr₂N, causing precipitation strengthening to increase the strength of steel. An excessively low content of Cr results in failure to provide the effects described above. In contrast, an excessively high content of Cr causes the generation of M₂₃C₆ carbides, resulting in decreases in the ductility and the toughness of steel. Consequently, the content of Cr is 15 to 30%. A lower limit of the content of Cr is preferably 20.5%, more preferably 21.0%. An upper limit of the content of Cr is preferably 23.5%, more preferably 23.4%.

N: 0.20 to 0.70%

Nitrogen (N) stabilizes austenite. In addition, N increases the strength of steel through solid-solution strengthening. Furthermore, N combines with Cr through heat treatment to form alloy carbo-nitrides such as Cr₂N, causing precipitation strengthening to increase the strength of steel. An excessively low content of N results in failure to provide the effects described above. In contrast, an excessively high content of N results in a decrease in the toughness of steel. Consequently, the content of N is 0.20 to 0.70%. A lower limit of the content of N is preferably 0.21%, more preferably 0.22%. An upper limit of the content of N is preferably 0.40%, more preferably 0.35%.

The balance of the chemical composition of the austenitic stainless steel material according to the present embodiment is Fe and impurities. Here, the impurities mean elements that are mixed from ores and scraps used as raw material, a producing environment, or the like, when the austenitic stainless steel material is produced in an industrial manner, and are allowed to be mixed within ranges in which the impurities have no adverse effects on the austenitic stainless steel material of the present embodiment.

[Optional Elements]

The austenitic stainless steel material according to the present embodiment may further contain, instead of a part of Fe, one, or two or more elements selected from the group consisting of Mo, V, and Nb. All of these elements increase the strength of steel.

Mo: 0 to 5.0%

Molybdenum (Mo) is an optional element and need not be contained. When contained, Mo subjects austenite to solid-solution strengthening. In addition, Mo increases the corrosion resistance of steel. However, an excessively high content of Mo is liable to cause intermetallic compounds to precipitate, results in a decrease in the ductility and the toughness of steel. Consequently, the content of Mo is 0 to 5.0%. A lower limit of the content of Mo is preferably 1.5%, more preferably 1.9%. An upper limit of the content of Mo is preferably 3.0%, more preferably 2.9%.

V: 0 to 0.5%

Vanadium (V) is an optional element and need not be contained. When contained, V forms its carbide, increasing the strength of steel. However, an excessively high content of V saturates the effect, resulting in an increase in production costs. Consequently, the content of V is 0 to 0.5%. A lower limit of the content of V is preferably 0.1%, more preferably 0.12%. An upper limit of the content of V is preferably 0.3%, more preferably 0.28%.

Nb: 0 to 0.5%

Niobium (Nb) is an optional element and need not be contained. When contained, Nb forms its carbide, increasing the strength of steel. However, an excessively high content of Nb saturates the effect, resulting in an increase in production costs. Consequently, the content of Nb is 0 to 0.5%. A lower limit of the content of Nb is preferably 0.1%, more preferably 0.12%. An upper limit of the content of Nb is preferably 0.3%, more preferably 0.28%.

[Strength and Strength Difference ΔTS]

In the austenitic stainless steel material of the present embodiment, a tensile strength is 800 MPa or more, and a difference between the maximum value and the minimum value of the tensile strength (hereafter, referred to as a strength difference ΔTS) is 50 MPa or less. This makes the austenitic stainless steel material of the present embodiment have a consistent high-strength across the overall length of the steel material. The above strength and strength difference ΔTS can be achieved with, for example, the following structure.

[Grain Size]

The austenitic stainless steel material of the present embodiment has a grain size number specified in ASTM E 112 of 6.0 or greater. The grain size number is measured in conformity with ASTM E 112. A grain size number less than 6.0 decreases the strength. With a grain size number of 6.0 or greater, it is possible to obtain a high strength in the austenitic stainless steel material having the above chemical composition. Specifically, a tensile strength of 800 MPa or more required for the austenitic stainless steel material of the present embodiment is obtained.

The grain size number is determined by the following method. A test specimen for microscopic observation is fabricated from a center portion of a cross section perpendicular to a lengthwise direction of the austenitic stainless steel material. In the surface of the test specimen, a surface corresponding to the above cross section (referred to as an observation surface) is used, and a microscopic test method for grain size specified in ASTM E 112 is performed, and the grain size number is evaluated. Specifically, the observation surface is subjected to mechanical polishing, and thereafter etched using a well-known etching reagent (e.g., Glyceregia, Kalling's reagent, or Marble's reagent), and crystal grain boundaries on the observation surface are caused to appear. For each of ten visual fields on the etched surface, a grain size number is determined. The area of each visual field is about 10.2 mm². By performing a comparison with a grain size standard chart specified in ASTM E 112, the grain size number of each visual field is evaluated. The average of the grain size numbers of the respective visual fields is defined as the grain size number of the austenitic stainless steel material of the present embodiment.

[Grain Size Difference ΔGS]

In addition, in the austenitic stainless steel material of the present embodiment, the difference between the maximum value and the minimum value of grain size numbers that are measured in any plurality of portions across overall length of the austenitic stainless steel material (referred to as the grain size difference ΔGS) is 1.5 or smaller. When the grain size difference ΔGS is more than 1.5, the difference between the maximum value and the minimum value of tensile strengths that are measured in a plurality of portions of the steel material (strength difference ΔTS) becomes larger than 50 MPa, and variation in strength across the overall length of the steel material becomes large. When the grain size difference ΔGS is 1.5 or smaller, the strength difference ΔTS becomes 50 MPa or less, and the variation in strength across the overall length of the steel material is suppressed. As a result, the austenitic stainless steel material of the present embodiment has a consistent high-strength.

The grain size difference ΔGS is measured by the following method. From any plurality of portions across the overall length of the austenitic stainless steel material in the lengthwise direction, the same test specimens for microscopic observation as those described above are taken. Using each of the test specimens, the microscopic test method for grain size specified in ASTM E 112 is performed in the same manner as described above, and the grain size number is determined. Of the obtained grain size numbers, a maximum value and a minimum value are selected, and the difference between the maximum value and the minimum value is defined as the grain size difference ΔGS. In a case where the austenitic stainless steel material is a steel pipe, a steel bar, a wire rod, or the like, the test specimens are taken from both end portions of the steel material in a hot working direction (rolling direction, extruding direction, or the like) (a top portion and a bottom portion), and the grain size difference ΔGS is determined. Here, the top portion is defined as a portion extending from a front end of the steel material toward the center portion of the steel material within a range of 200 mm, and the bottom portion is defined as a portion extending from a rear end of the steel material toward the center portion of the steel material within a range of 200 mm.

The smaller the grain size difference ΔGS, the more preferable it is. An upper limit of the grain size difference ΔGS is preferably 1.3, more preferably 1.0.

[Alloy Carbo-Nitrides]

By performing heat treatment on a steel material to cause coarse alloy carbo-nitrides to precipitate, precipitation strengthening occurs, increasing the strength of the steel material.

The alloy carbo-nitrides contain Cr, V, Nb, Mo, W, Ta, or the like as a main component and include Cr₂N, Z phase, namely Cr(Nb, V)(C, N) and MX (M: Cr, V, Nb, Mo, W, Ta, etc., X: C, N). In addition, the carbo-nitrides in the present invention include those of which contents of C (carbon) are extremely low, namely nitrides. The carbo-nitrides in the present invention also include carbides.

In the present embodiment, the number of alloy carbo-nitrides having a circle equivalent diameter of larger than 1000 nm (coarse alloy carbo-nitrides) in the steel is 10/mm² or more. In this case, it is possible to obtain a high tensile strength through the precipitation strengthening. If the number of the coarse alloy carbo-nitrides is excessively large, the toughness of the steel may decrease, and thus an upper limit of the number of the coarse alloy carbo-nitrides in the steel is preferably 1.5×10⁵/mm². By performing the heat treatment with a heat treatment temperature set at 930° C. to less than 1000° C., it is possible to obtain 10/mm² or more of the coarse alloy carbo-nitrides.

[Method for Measuring Number of Coarse Alloy Carbo-Nitrides]

The number of the coarse alloy carbo-nitrides is defined as follows. A sample is taken that includes a center portion of a cross section of the austenitic stainless steel material, the cross section being perpendicular to the lengthwise direction of the austenitic stainless steel material (an observation region having a radius of 10 mm around the central axis of the steel material). The above observation region of the sample is subjected to mirror polish. Thereafter, in each of any ten visual fields (200 μm×200 μm) in the observation region, alloy carbo-nitrides are identified from precipitates and inclusions using a scanning electron microscope (SEM) equipped with an energy-dispersive X-ray spectroscope (EDS). In each visual field, a circle equivalent diameter of each of the identified alloy carbides is determined by image analysis. The circle equivalent diameter means a diameter (nm) of a circle into which the area of an alloy carbide in the visual field is converted. The number of alloy carbo-nitrides having circle equivalent diameters of larger than 1000 nm (coarse alloy carbo-nitrides) is counted. The average value of the coarse alloy carbo-nitrides obtained in each of the ten visual fields is defined as the number of coarse alloy carbo-nitrides (/mm²) in the present specification.

[Producing Method]

Description will be made about an example of a production method for an austenitic stainless steel material according to the present embodiment. The present producing method includes a preparation step of preparing a starting material, a hot working step of performing hot working on the starting material to produce an intermediate material, a cooling step of cooling the intermediate material subjected to the hot working, and as necessary, a heat treatment step of performing heat treatment on the cooled intermediate material. The producing method will be described below.

[Preparation Step]

A molten steel having the chemical composition described above is produced. As necessary, a well-known degassing treatment is performed on the produced molten steel. From the molten steel subjected to the degassing treatment, a starting material is produced. Examples of the producing method for the starting material include a continuous casting process. By the continuous casting process, a continuous casting material (the starting material) is produced. The continuous casting material is, for example, a slab, a bloom, a billet, and the like. The molten steel may be subjected to an ingot-making process into an ingot.

[Hot Working Step]

The starting material (continuous casting material or ingot) is subjected to hot working by a well-known method to be produced into the intermediate material of the austenitic stainless steel material. Examples of the intermediate material include a steel pipe, a steel bar, a wire rod, and the like. The intermediate material is produced by, for example, hot extrusion working according to the Ugine-Sejoumet process.

In the hot working step, a heating temperature and a reduction of area are as follows.

Heating Temperature: 1160° C. or Less

An excessively high heating temperature causes grains to coarsen, with the result that the grain size number of structures of the steel becomes less than 6.0. Therefore, the heating temperature is 1160° C. or less. An upper limit of the heating temperature is preferably 1100° C.

A lower limit of the heating temperature may be a well-known temperature. An excessively low heating temperature makes the coarse alloy carbo-nitrides hard to be generated even with the heating treatment to be described later that is performed after the hot working. Consequently, a lower limit of the heating temperature is preferably 1060° C.

Reduction of Area: Larger than 70%

When the cross-sectional area of the starting material before the hot working is denoted by A0 (mm²), and the cross-sectional area of the starting material after the final operation of the hot working is denoted by A1 (mm²), a reduction of area RA (%) is defined by Formula (1).

RA=(A0−A1)/A0×100  (1)

When the above reduction of area is 70% or less, an amount of strains to be introduced into the steel material becomes insufficient, and grains are hard to be refined. When the reduction of area is 70% or more, an adequate amount of strains is introduced into the steel material through the hot working, which refines the grains, making the grain size number 6.0 or greater. A lower limit of the reduction of area is preferably 75%.

Temperature Difference ΔT in the Starting Material in the Hot Working: 100° C. or Less

In the hot working step, in the starting material, the difference between the temperature of a portion for which the hot working is first completed, at the completion of the hot working, (referred to as the initial temperature) and the temperature of a portion for which the hot working is last completed, at the completion of the hot working, (referred to as the end temperature) (temperature difference ΔT) is 100° C. or less.

For example, in a case where the intermediate product is produced by performing piercing-rolling, hot extrusion, and hot rolling, the portion for which the hot working is first completed of the starting material is a top portion, and the portion for which the hot working is last completed of the starting material is a bottom portion. Therefore, in this case, the initial temperature is a temperature of the top portion at the completion of the hot working, and the end temperature is a temperature of the bottom portion at the completion of the hot working.

When the temperature difference ΔT of the starting material is larger than 100° C., variation in temperature across the overall length of the steel material becomes excessively large. In this case, the grain size of the top portion and the grain size of the bottom portion differ from each other significantly, and the grain size difference ΔGS becomes larger than 1.5. As a result, the strength difference ΔTS becomes larger than 50 MPa.

When the temperature difference ΔT of the starting material is 100° C. or less, variation in grain size between the top portion and the bottom portion is suppressed, and the grain size difference ΔGS becomes 1.5 or less. As a result, the strength difference ΔTS becomes 50 MPa or less. An upper limit of the temperature difference ΔT is preferably 90° C., more preferably 80° C.

[Cooling Step]

In the cooling step, the intermediate product subjected to the hot working is cooled at 0.10° C./sec or more. When the cooling rate is less than 0.10° C./sec, a phases precipitate. The σ phases decrease corrosion resistance. To increase the corrosion resistance, the generation of σ phases needs to be suppressed. Furthermore, when the cooling rate is less than 0.10° C./sec, grains coarsen, which decreases the strength of the steel. Consequently, the cooling rate is 0.10° C./sec or more.

On the intermediate product after the cooling, straightening may be performed to straighten the bends of the intermediate product. In a case of performing the straightening, for example, a straightener is disposed in line or off line on a downstream side of a cooling device and/or an upstream side of a heating device.

On the intermediate product subjected to the cooling or the straightening, descaling treatment may be performed. The descaling treatment is performed in the form of, for example, pickling or shotblasting. The descaling treatment is performed to remove oxide scale that is unavoidably formed on the surface of the intermediate product due to being heated in the step prior thereto. Through the above steps, the austenitic stainless steel material of the present embodiment is produced.

[Heat Treatment Step]

In the heat treatment step, 10/mm² or more of coarse alloy carbo-nitrides are caused to precipitate. Through this step, the tensile strength of the austenitic stainless steel material is further increased. A heat treatment temperature is as follows.

Heat Treatment Temperature: 930° C. to Less than 1000° C.

A heat treatment temperature less than 930° C. leads to a failure to obtaining structures of an austenite single phase, resulting in a decrease in strength. The heat treatment temperature less than 930° C. further leads to the generation of σ phases, resulting in a decrease in the corrosion resistance of steel. In contrast, a heat treatment temperature of 1000° C. or more causes coarse alloy carbo-nitrides in steel to become smaller or totally dissolved, and the number of coarse alloy carbo-nitrides becomes less than 10/mm². As a result, precipitation strengthening cannot be obtained.

A heat treatment temperature of 930° C. to less than 1000° C. causes coarse alloy carbo-nitrides to precipitate, and the number of the coarse alloy carbo-nitrides becomes 10/mm² or more. As a result, precipitation strengthening occurs, increasing the strength of the steel material. In addition, a heat treatment temperature less than 1000° C. causes coarse alloy carbo-nitrides to precipitate sufficiently, with the result that a strength of 800 MPa or more is obtained with stable also when the grain size number is within a range of 6.0 to less than 8.0.

Note that, even if the heat treatment temperature falls out of the above range, it is possible to obtain a high strength as long as the grain size number is 6.0 or greater, and the number of the coarse alloy carbo-nitrides in steel is 10/mm² or more, and it is possible to control the strength difference ΔTS to 50 MPa or less as long as the grain size difference ΔGS is 1.5 or smaller.

A retention time in the heat treatment at the above heat treatment temperature is, for example but not specially limited to, one minute or longer.

The production method according to the present embodiment may include a cold working step of performing cold working, after the heat treatment step. However, solid solution heat treatment is not performed after the cold working step because there is the possibility of failing to obtain coarse alloy carbo-nitrides.

Examples

Molten steels having chemical compositions shown in Table 1 were produced.

TABLE 1 Chemical Composition (In Mass %, Balance Being Fe and Impurities) Steel C Si Mn P S Ni Cr N Mo V Nb A 0.03 0.3 4.5 0.02 0.0003 12.0 22.3 0.32 2.1 0.2 0.2 B 0.03 0.4 4.4 0.01 0.0008 12.1 21.9 0.32 2.1 0.2 0.2 C 0.01 0.3 5.2 0.02 0.0002 13.1 22.0 0.32 2.2 0.2 0.2 D 0.01 0.4 5.3 0.01 0.0004 12.9 21.8 0.31 — — — E 0.01 0.3 5.2 0.02 0.0020 13.2 22.0 0.32 — — 0.1 F 0.03 0.4 4.5 0.02 0.0020 15.6 22.1 0.19 2.1 0.1 0.1

Using the molten steels, ingots each weighing 3400 kg were produced. The ingots were subjected to the hot working to be produced into austenitic stainless steel bars (intermediate products) (45 to 75 mm in diameter×3000 mm in length). In the hot working, initial temperatures (temperatures of top portions at the completion of hot extrusion), end temperatures (temperatures of bottom portions at the completion of the hot extrusion), and reductions of area RA (%) were those shown in Table 2.

TABLE 2 Heat Heating Initial End Temperature Reduction Cooling Treatment Test Temperature Temperature Temperature Difference of Area Rate Temperature Number Steel (° C.) (° C.) (° C.) ΔT (° C.) RA (%) (° C./Sec) (° C.)  1 B 1102 1102 1003 99 80.2 1 993  2 B 1038 1038 1038 0 88.9 0.7 973  3 D 1150 1150 1101 49 76 1.1 994  4 E 1150 1150 1101 49 76 0.2 979  5 A 1210 1210 1161 49 91.4 2.1 960  6 A 1180 1180 1147 33 89.2 0.2 1002  7 B 1162 1162 1113 49 93.8 0.5 984  8 C 1150 1150 709 441 12 1 1029  9 C 1150 1150 1134 16 10 0.7 985 10 C 1150 1150 1117 33 90 0.07 980 11 C 1130 1130 1048 82 85 0.3 600 12 C 1150 1150 1117 33 97 0.9 1200 13 F 1150 1150 1150 0 76 0.4 1031 14 A 1140 1140 1124 16 71 0.1 1100 15 B 1093 1093 1060 33 76 0.5 1097 16 C 1157 1157 1013 144 72 0.4 960 17 C 1143 1143 1021 122 71 1.2 1046 18 A 1150 1150 1101 49 76 1 — Number of Grain Coarse Size Grain Size Alloy Top Bottom Strength Number Number of Grain size Carbo- Portion Portion Difference Average Test of Top Bottom Difference Nitrides TS TS ΔTS TS Number Portion Portion ΔGS (/mm²) (MPa) (MPa) (MPa) (MPa)  1 7.6 8.3 0.7 141 821 840.8 19.8 830.9  2 8.9 8.9 0 183 860.7 857.4 3.3 859.1  3 7 7.3 0.3 139 804.5 811.3 6.8 807.9  4 7.1 7.3 0.2 170 808.4 817.7 9.3 813.1  5 4.3 4.6 0.3 210 751.8 762.3 10.5 757.1  6 4.6 4.8 0.2 122 757.8 778.5 20.7 768.2  7 5.8 6.1 0.3 160 773.8 800.1 26.3 787  8 3.6 5.3 1.7 65 738.2 790.3 52.1 764.3  9 5.8 5.9 0.1 158 770.3 777.1 6.8 773.7 10 4.8 5.2 0.4 156 777.5 793.2 15.7 785.4 11 7.2 7.7 0.5 156400 792.7 799 6.3 795.9 12 4 4.2 0.2 0 741.6 743 1.4 742.3 13 6.1 6.1 0 61 730.4 739.3 8.9 734.9 14 6.2 6.3 0.1 0 762.7 768.9 6.2 765.8 15 6.5 6.7 0.2 0 768.3 787.1 18.8 777.7 16 6 8.8 2.8 210 802 860.7 58.7 831.4 17 6.1 8.9 2.8 29 800.5 862.1 61.6 831.3 18 6.3 6.8 0.5 0 782.4 795.2 12.8 788.8

The produced material pipes were cooled at cooling rates shown in Table 2. In addition, on the cooled material pipes, the straightening and the descaling treatment were performed. Furthermore, the heat treatment was performed at heat treatment temperatures shown in Table 2 to produce austenitic stainless steel materials (steel pipes). The retention time was 45 minutes. On a test number 18, the heat treatment was not performed. Note that the tensile strength (the grain size) is greatly influenced by a work completion temperature in the hot working, there are a tendency for the top portion, at a high temperature, to have a high strength (small grain size) and a tendency for the bottom portion, at a low temperature, to have a low strength (large grain size). For that reason, a maximum value and a minimum value of tensile strength were measured for each of the top portion and the bottom portion.

[Measuring Grain Size Number]

Using test specimens taken from a top portion and a bottom portion of the produced steel material of each test number subjected to the hot working, a grain size test was conducted based on the above ASTM E 112. The samples were taken from positions corresponding to the top portion and the bottom portion of each steel material (wall-thickness center portions). The grain size numbers of the top portion and the bottom portion were determined, and in addition, the grain size difference ΔGS was determined. The resultant grain size numbers and grain size differences ΔGS are shown in Table 2.

[Counting Number of Coarse Alloy Carbo-Nitrides]

From a wall-thickness center portion of the steel material of each test number, a test specimen was taken. Using the taken test specimen, the number of coarse alloy carbo-nitrides (/mm²) was determined by the above method.

[Tension Test]

From center portions of the top portion and the bottom portion of the steel material of each test number, a round-bar tensile test specimen was taken. The round-bar tensile test specimen includes a wall-thickness center portion of the steel material (steel pipe), and a parallel portion of the round bar specimen was parallel to a lengthwise direction of the steel material. The diameter of the parallel portion was 5 mm. Using the round bar specimen, in conformance with JIS Z2241(2011), a tension test was performed in the atmosphere at a normal temperature (25° C.), and tensile strengths TS (MPa) of the top portion and the bottom portion of each test number were determined. In addition, the strength difference ΔTS (MPa) was determined for each test number.

[Test Results]

Table 2 shows test results.

Referring to Table 2, as to the steels of test numbers 1 to 4, their chemical compositions and producing conditions were appropriate. As a result, the grain size numbers were 6.0 or greater, and the grain size differences ΔGS were 1.5 or smaller. Furthermore, the numbers of coarse alloy carbo-nitrides were 10/mm² or more. Therefore, the tensile strengths were as high as 800 MPa or more, in addition, the strength differences ΔTS were 50 MPa or less, and thus consistent high-strengths were obtained across the overall lengths of the steel materials.

In contrast, as to test numbers 5 to 7, their chemical compositions were appropriate, but their heating temperatures in the hot working were excessively high. Therefore, the grain size numbers of the top portion and/or the bottom portion were less than 6.0. As a result, the strengths of the steels were less than 800 MPa, indicating low strengths.

As to a test number 8, its chemical composition was appropriate, but its temperature difference ΔT in the hot working was larger than 100° C., and its reduction of area was less than 70%. Therefore, the grain size number thereof was less than 6.0, and the grain size difference ΔGS was larger than 1.5. As a result, the strength of the steel was less than 800 MPa, indicating a low strength. In addition, the strength difference ΔTS was larger than 50 MPa, indicating a large variation in strength.

As to a test number 9, its chemical composition was appropriate, but its reduction of area in the hot working was less than 70%. Therefore, the grain size number was less than 6.0. As a result, the tensile strength was less than 800 MPa, indicating a low strength.

As to a test number 10, its chemical composition was appropriate, but its cooling rate after the hot working was less than 0.10° C./sec. Therefore, the grain size number was less than 6.0. As a result, the strength of the steel was less than 800 MPa, indicating a low strength.

As to a test number 11, its chemical composition was appropriate, but its heat treatment temperature after the cooling was less than 930° C. As a result, the strength of the steel was less than 800 MPa, indicating a low strength.

As to a test number 12, its chemical composition was appropriate, but its heat treatment temperature after the cooling was as excessively high as 1200° C. For that reason, the number of coarse alloy carbo-nitrides was less than 10/mm², and the grain size number was less than 6.0. As a result, the tensile strength was less than 800 MPa.

As to a test number 13, the content of N was excessively low. As a result, the tensile strength was less than 800 MPa.

As to test numbers 14 and 15, their chemical compositions were appropriate, but their heat treatment temperatures after the cooling were 1000° C. or more. Therefore, the numbers of coarse alloy carbo-nitrides were less than 10/mm². As a result, the tensile strengths were less than 800 MPa.

As to test numbers 16 and 17, their chemical compositions were appropriate, but their temperature differences ΔT of the steel materials in the hot working were larger than 100° C. Therefore, the grain size differences ΔGS were more than 1.5. As a result, the strength differences ΔTS were more than 50 MPa, indicating large variations in strength.

In the test number 18, the heat treatment was not performed. Therefore, no coarse alloy carbo-nitrides were present. As a result, the tensile strength was less than 800 MPa.

As described above, the embodiment according to the present invention has been described. However, the aforementioned embodiment is merely an example for practicing the present invention. Therefore, the present invention is not limited to the aforementioned embodiment, and the aforementioned embodiment can be modified and implemented as appropriate without departing from the scope of the present invention. 

1. An austenitic stainless steel material comprising: a chemical composition consisting of, in mass percent: C: 0.10% or less; Si: 1.0% or less; Mn: 3 to 8%; P: 0.05% or less; S: 0.03% or less; Ni: 10 to 20%; Cr: 15 to 30%; N: 0.20 to 0.70%; Mo: 0 to 5.0%; V: 0 to 0.5%; and Nb: 0 to 0.5%, with the balance being Fe and impurities, wherein a grain size number conforming to ASTM E 112 is 6.0 or greater, a tensile strength is 800 MPa or more, a difference between a maximum value and a minimum value of the tensile strength is 50 MPa or less, and a number of alloy carbo-nitrides with circle equivalent diameters of larger than 1000 nm in steel is 10/mm² or more.
 2. The austenitic stainless steel material according to claim 1, wherein the chemical composition contains one, or two or more elements selected from the group consisting of: Mo: 1.5 to 5.0%; V: 0.1 to 0.5%; and Nb: 0.1 to 0.5%.
 3. The austenitic stainless steel material according to claim 1, wherein a difference between a maximum value and a minimum value of the grain size number is 1.5 or smaller.
 4. The austenitic stainless steel material according to claim 1, wherein the austenitic stainless steel material is one of a steel pipe, a steel bar, and a wire rod.
 5. The austenitic stainless steel material according to claim 2, wherein a difference between a maximum value and a minimum value of the grain size number is 1.5 or smaller.
 6. The austenitic stainless steel material according to claim 2, wherein the austenitic stainless steel material is one of a steel pipe, a steel bar, and a wire rod.
 7. The austenitic stainless steel material according to claim 3, wherein the austenitic stainless steel material is one of a steel pipe, a steel bar, and a wire rod.
 8. The austenitic stainless steel material according to claim 4, wherein the austenitic stainless steel material is one of a steel pipe, a steel bar, and a wire rod. 